High Total Transmittance Alumina Discharge Vessels Having Submicron Grain Size

ABSTRACT

The present invention uses a post-sinter-HIP anneal to increase the total transmittance of ceramic discharge vessels comprised of a submicron-grained alumina doped with MgO. After the anneal, the submicron-grained alumina discharge vessels have high values of both total and in-line transmittance, and are thus suitable for use in focused-beam, short-arc lamps. In particular, the total transmittance of the discharge vessel is increased to greater than 92% in the wavelength range from about 400 nm to about 700 nm.

BACKGROUND OF THE INVENTION

Translucent polycrystalline alumina (PCA) ceramic has made possiblepresent-day high-pressure sodium (HPS) and ceramic metal halide lamps.The arc discharge vessels in these applications must be capable ofwithstanding the high temperatures and pressures generated in anoperating lamp as well be resistant to chemical attack by the fillmaterials. In addition, the discharge vessels are typically required tohave >92% total transmittance in the visible wavelength region fromabout 400 nm to about 700 nm in order to be useable in commerciallighting applications.

In HPS lamps, the discharge vessels are tubular, whereas for ceramicmetal-halide lamps discharge vessels can range from a cylindrical shapeto an approximately spherical shape (bulgy). Examples of these types ofarc discharge vessels are given in European Patent Application No. 0 587238 A1 and U.S. Pat. No. 5,936,351, respectively. The bulgy shape withits hemispherical ends yields a more uniform temperature distribution,resulting in reduced corrosion of the PCA by the lamp fills.

Because PCA is translucent and not transparent, the use of PCA islimited to non-focused-beam lamp applications. Birefringent grainscattering is a major source of in-line transmittance loss in regular,sintered PCA, and in-line transmittance generally increases withincreasing grain size. Reducing the grain size of sinter-HIPed PCA tothe submicron range (<1 micrometer) shifts the scattering mechanismthereby decreasing grain birefringent scattering. In the submicronregion, the in-line transmittance actually increases with decreasinggrain size. The high in-line transmittance and mechanical strength ofsubmicron-grained PCA are of interest for focused-beam, short-arc lamps,that offer improved luminance, efficacy, and color rendition.

Magnesia (MgO) is typically required as a sintering aid in themanufacture of alumina discharge vessels in order to retard grain growthand facilitate grain boundary diffusion while pinning grain boundaries.Submicron alumina ceramics based on nano-sized starting powdersgenerally require a higher level MgO to reach full density thanlarger-grained (10-30 microns) alumina based on micron-sized startingpowders. This is because nano-sized powder requires a higher level ofMgO dopant to cover the surface of the finer particles. Moreover, unlikethe larger-grained alumina, the MgO-based dopants (e.g. ˜200-300 ppm)become completely dissolved in lattice and grain boundary region. As aresult, high levels of color centers can form including a variety ofsingle and double oxygen vacancies with one or two electrons. Thesecolor centers absorb light which result in a low total transmittance(˜78%) for MgO-doped, submicron-grained alumina discharge vesselsdespite their high in-line transmittance.

SUMMARY OF THE INVENTION

The present invention overcomes the problem of low total transmittancein MgO-doped, submicron-grained alumina discharge vessels by apost-sinter-HIP anneal. The anneal is believed to alter the ionizationand association states of the color centers, but not significantlychange the total population of oxygen vacancies. The temperature, time,and partial pressure of oxygen of the annealing atmosphere arecontrolled so that significant portions of light-absorbing oxygenvacancies are converted to non-light-absorbing oxygen vacancies while atthe same time a stable microstructure is maintained (no significantgrain growth or precipitation of magnesium aluminate spinel second phaseparticles). As a result, the post-sinter HIP anneal effectively bringsthe total transmittance up to >92% in the range from about 400 nm toabout 700 nm, a level suitable for commercial lamp applications.

In accordance with an aspect of the invention, there is provided amethod of making a ceramic discharge vessel comprising: (a) forming thedischarge vessel with a submicron-grained alumina powder doped with MgO;(b) sinter-HIPing the discharge vessel; and (c) annealing the dischargevessel to increase the total transmittance of the discharge vesselto >92% in a wavelength range from about 400 nm to about 700 nm.

In accordance with another aspect of the invention, there is provided aceramic discharge vessel comprising a ceramic body comprised of asubmicron-grained alumina doped with MgO, the discharge vessel having atotal transmittance of greater than 92% in a wavelength range from about400 nm to about 700 nm.

It is expected that additional co-doping of the PCA with ZrO₂, Y₂O₃,Er₂O₃, Yb₂O₃, Sc₂O₃, etc., may alter the relative population of thepoint defects, but the problem of low total transmittance is expected tostill exist because of the presence of MgO. Thus, the method of thisinvention should be useful to increase the total transmittance of suchco-doped alumina ceramics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration of a bulgy-shaped dischargevessel according to this invention.

FIG. 2 is a schematic illustration of an apparatus for measuring thetotal transmittance of a discharge vessel.

FIG. 3 is a magnified view of the tapered end of the optical fiber usedin the apparatus shown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawings.

FIG. 1 is a cross-sectional illustration of an arc discharge vesselaccording to this invention. The arc discharge vessel 21 has a ceramicbody 23 which is comprised of submicron-grained polycrystalline aluminadoped with MgO. The ceramic body 23 has a total transmittance of greaterthan 92% in the wavelength range from about 400 nm to about 700 nm. Morepreferably, the total transmittance is greater than 95% over the samewavelength range. The body 23 defines an arc discharge cavity 25 and hastwo capillaries 27 extending outwardly in opposite directions from thedischarge cavity 25. Preferably, the thickness of the discharge cavitywall is about 0.8 mm. The capillaries are suitable for receiving, andsealing therein, electrode assemblies (not shown) which provide aconductive path for supplying electric power to the discharge vessel inorder to strike and sustain an arc within the discharge cavity. Althoughthe embodiment shown in FIG. 1 is a bulgy-shaped arc discharge vessel,other suitable shapes for the arc discharge vessel of this inventioninclude tubular arc discharge vessels similar to HPS arc tubes.

Ceramic discharge vessels formed from a high-purity, finely dividedaluminum oxide (alumina) powder may be consolidated by isopressing, slipcasting, gel casting, or injection molding. The MgO dopant is generallyadded to the alumina powder prior to consolidation. Preferably, the MgOdopant in the alumina powder ranges from about 200 ppm to about 800 ppmand the alumina powder has a mean particle size of about 150 nm. Thedetails of various methods of manufacturing green ceramic bodies fordischarge vessels are described in, for example, European Patent No. 0650 184 B1 (slip casting), U.S. Pat. No. 6,399,528 (gel casting),International Patent Application No. WO2004/007397 A1 (slip casting) andEuropean Patent Application No. EP 1 053 983 A2 (isopressing).

In the case of single-piece, hollow-core vessels, such as thebulgy-shaped discharge vessel, a fugitive core may be used inconjunction with a shaping (e.g., gel-casting) method to form thedischarge vessel. To remove the core after shaping, the green bodycontaining the fugitive core (such as wax) is heated slowly to themelting point of the core (˜50° C. for wax), allowing the molten corematerial to drip and flow out of the inside cavity. In a preferredmethod, the binder materials are removed by slow heating in flowing airto 800° C. Too fast a heating rate may cause binder entrapment insidethe densified powder compact, which causes the final sinter-HIPed sampleto have a gray hue. Presintering is conducted at 1250-1270° C. for 2 hin air to reach a state of closed porosity. The presintered parts arehot isostatically pressed (HIPed) and sintered at 1270° C. under ˜10 ksiargon.

The total transmittance of the discharge vessels can be measured byilluminating one end of a small light pipe with a tungsten halogen lamp,placing the other end inside the hollow body, and measuring the totalintegrated flux of light passing out of the discharge vessel over thewavelength range from about 400 nm to about 700 nm. A schematicillustration of an apparatus for measuring the total transmittance isshown in FIG. 2. The sample 11 (in this case a tubular vessel) ismounted in a center region of integrating sphere 3 by supports 9 whichhold the sample at opposite ends. A baffle 13 is positioned between thesample 11 and light detector 15. Optical fiber 7 is inserted through oneof the supports 9 and into a center portion of the sample 11. Light isconducted from tungsten-halogen lamp 5 through the optical fiber 7. Asshown in FIG. 3, the light-emitting end 17 of the optical fiber 7, whichis placed inside the sample, has a 20° taper with respect to thecylindrical axis of the fiber in order to scatter the emitted light toproduce a nearly point source. The light emitted from the end of theoptical fiber passes through the sample wall, is collected by theintegrating sphere, and the total integrated flux is measured by thelight detector, preferably an unfiltered silicon detector. Thepercentage of the total integrated flux transmitted through the sample(compared to the total integrated flux emitted by the optical fiberalone) represents the total transmittance of the sample. Since thetransmittance of the plastic optical fiber falls off at either end ofthe visible spectrum, i.e., less than about 400 nm and greater thanabout 700 nm, the total transmittance is effectively measured in thewavelength range from about 400 nm to about 700 nm. Suitable componentsfor the apparatus are available from Labsphere, Inc. of North Sutton,N.H.

EXAMPLES

Both disks (˜25 mm diameter by 0.8 mm thick) and 70 W bulgy-shapeddischarge vessels were made by sinter-HIPing. The startingsubmicron-grained alumina powder had a 150 nm mean particle size and wasdoped with 220 ppm MgO. For the disks, the measured in-linetransmittance was typically quite uniform within the same disk, and fromdisk to disk. The as-made, sinter-HIPed bodies had very few residualpores. The average grain size was about 0.5 microns. Grain size wasdetermined by multiplying a factor of 1.5 to the intercept size measuredusing images acquired via scanning electron microscopy (SEM).

The total transmittance values measured for the sinter-HIPed dischargevessels (polished or non-polished with a 0.8-mm wall thickness) wererelatively low, ranging from 69-87% with an average of about 77%. Thespectrophotometer in-line transmittance of polished 0.8 mm-thick disksat 600 nm was measured as 50-55%. After annealing at 1025-1150° C. inair for various durations (2-4 h), the total transmittance ofsinter-HIPed, submicron-grained alumina vessels increased to 87-98%(Table 1). The scatter in the total transmittance may be attributed tothe different levels of point defects, brought about by alocation-specific, partial pressure of oxygen in the hot isostaticpress. Only about 10 out of 30 annealed vessels were fairly colorless,while most of them were brownish. This undesirable brown color isbelieved to be related to point defects. The brown color was one reasonthat air anneal could not increase the total transmittance to >92% for anumber of vessels.

The total transmittance remained unchanged even after re-firing thevessels in wet hydrogen at 1100° C. for 2 h. Since wet hydrogen firingtypically increases the transmittance of dry-hydrogen (very lowP_(O2))-sintered PCA, the fact that the total transmittance remainedunchanged in this case indicates that the partial pressure of oxygenduring HIPing was probably even more reducing than a dry H₂ environment(P_(O2)<1×10⁻¹² atm).

Annealing several of the air-annealed samples under a Ar-5 ppm O₂atmosphere (P_(O2)=5×10⁻⁶ atm) at 1100° C. for 2 hours increased thetotal transmittance of the submicron-grained discharge vessels from87-96% to 91-97% as shown in Table 1 (in-line transmittance is given inparentheses). These samples also became visually less colored. Thismeans that, in terms of minimizing light-absorbing color centers, aP_(O2) of 5×10⁻⁶ atm in the anneal atmosphere is much better than thatof air (P_(O2)˜0.2 atm) or wet hydrogen ((P_(O2)<1×10⁻¹² atm).

The P_(O2) and temperature ranges in the post-sinter-HIP anneal aredictated by the thermodynamics of the point defects and their diffusionrates, whereas the annealing time and MgO levels depend on the thicknessof the final part and the particle size of the starting powder.Preferably, the annealing atmosphere is an inert gas, e.g., nitrogen orargon, that has a partial pressure of oxygen from about 1×10⁻³ atm toabout 1×10⁻⁹ atm, and more preferably about 1×10⁻⁵ atm to about 1×10⁻⁷atm.

If the anneal temperature is too high, spinel precipitates will form,in-line transmission loss occurs, and grains/residual pores grow to makethe parts opaque. The annealing temperature may range preferably fromabout 1000° C. to about 1150° C. and the annealing time preferably mayrange from about 1 hour to about 20 hours. In a more preferredembodiment, the annealing temperature is about 1100° C. and theannealing time is about 2 hours. TABLE 1 Total Transmittance (%) (andIn-line Transmittance (%)) Before and After Anneals After Air Ar- 5 ppmO₂ Samples As-made Anneal Anneal As-HIPed and polished 89.5 (2.03) 95.6(1.85) 97.4 (2.02) As-HIPed and polished 81.3 (3.68) 93.4 (3.22) 96.1(3.60) As-HIPed 73.1 (2.32) 90.6 (2.22)   93 (2.04) As-HIPed 73.6 (3.5) 88.8 (2.90) 94.3 (3.01) As-HIPed 71.4 (2.94) 89.9 (2.87) 94.1 (2.96)As-HIPed 69.1 (3.35) 89.2 (3.38) 94.8 (3.42) As-HIPed 74.4 (2.39) 89.4(2.49)   96 (2.27) As-HIPed 69.5 (2.16) 88.9 (2.39)   94 (2.09) As-HIPed76.1 (3.88) 90.3 (2.31) 96.4 (3.39) As-HIPed 77.6 (2.49)   89 (2.52)93.2 (2.40) As-HIPed 72.6 (3.34) 88.5 (3.39) 94.4 (4.09) As-HIPed andpolished 84.2 (3.82) 91.5 (3.83) 96.5 (3.66) As-HIPed 72.5 (2.24)   87(3.10) 92.3 (3.72) As-HIPed 72.7 (3.46) 89.6 (3.41) 95.5 (3.05) As-HIPed78.7 (3.54) 87.6 (2.71) 91.4 (2.67) As-HIPed and polished   88 (4.23)  93 (3.90) 96.9 (4.6)  As-HIPed and polished 85.8 (4.34) 92.7 (4.43)97.2 (5.06) As-HIPed and polished 88.4 (6.52) 92.6 (5.00) 96.7 (5.60)

UHP-grade N₂ gas typically contains 0.1-1 ppm oxygen (P_(O2)=0.1-1×10⁻⁶atm), and as such should be a favorable gas for anneal. Table 2 showsthat after annealing at 1100° C. for 2 h in nitrogen, the totaltransmittance increased to >92%. Moreover, it was shown in anotherexperiment that a direct anneal for 20 hours at 1150° C. in UHP-grade N₂of an as-sinter-HIPed, submicron-grained discharge vessel increasedtotal transmittance from 74.5% to 97.2%. TABLE 2 Total Transmittance (%)(and In-line Transmittance (%)) Before and After Various Anneals AfterAfter As sinter- After Air Ar-5 ppm UHP-grade Samples HIPed Anneal O₂Anneal N₂ Anneal As-HIPed 73.1 (2.32) 90.6 (2.22)   93 (2.04) 93.9(4.40) As-HIPed 78.7 (3.54) 87.6 (2.71) 91.4 (2.67) 92.2 (2.56) As-HIPedand 82.9 (3.74) 92.5 (3.65) 96.7 (4.05) 98.1 (4.16) polished As-HIPedand 87.5 (4.23) 92.3 (4.47) 96.1 (4.30) 97.5 (4.40) polished As-HIPed65.9 (1.61) 97.3 (1.98) — 97.4 (2.24)

The data clearly show the benefits of post-sinter-HIP anneal inincreasing the total transmittance to >92%, the level required forcommercial lighting applications. The anneal temperature and timeconform to the temperature limit required for microstructural andtransparency (in-line transmittance) stability.

While there have been shown and described what are present considered tobe the preferred embodiments of the invention, it will be apparent tothose skilled in the art that various changes and modifications can bemade herein without departing from the scope of the invention as definedby the appended claims.

1. A method of making a ceramic discharge vessel comprising: (a) formingthe discharge vessel with a submicron-grained alumina powder doped withMgO; (b) sinter-HIPing the discharge vessel; and (c) annealing thedischarge vessel to increase the total transmittance of the dischargevessel to >92% in a wavelength range from about 400 nm to about 700 nm.2. The method of claim 1 wherein the discharge vessel is annealed atabout 1000° C. to about 1150° C.
 3. The method of claim 2 where thedischarge vessel is annealed for about 1 hour to about 20 hours.
 4. Themethod of claim 1 wherein the discharge vessel is annealed at about1100° C. for about 2 hours.
 5. The method of claim 1 wherein thedischarge vessel is annealed in an atmosphere containing a partialpressure of oxygen of about 1×10⁻³ atm to about 1×10⁻⁹ atm.
 6. Themethod of claim 5 wherein the partial pressure of oxygen is about 1×10⁻⁵atm to about 1×10⁻⁷ atm.
 7. The method of claim 5 wherein the dischargevessel is annealed at about 1000° C. to about 1150° C.
 8. The method ofclaim 7 where the discharge vessel is annealed for about 1 hour to about20 hours.
 9. The method of claim 1 wherein the discharge vessel isformed with a submicron-grained alumina powder doped with about 200 ppmto about 800 ppm MgO.
 10. The method of claim 8 wherein the dischargevessel is formed with a submicron-grained alumina powder doped withabout 200 ppm to about 800 ppm MgO.
 11. The method of claim 9 whereinthe submicron-grained alumina powder has a mean particle size of about150 nm.
 12. The method of claim 5 wherein the annealing atmosphere iscomprised of nitrogen or argon.
 13. The method of claim 10 wherein theannealing atmosphere is comprised of nitrogen or argon.
 14. A ceramicdischarge vessel comprising: a ceramic body comprised of asubmicron-grained alumina doped with MgO, the discharge vessel having atotal transmittance of greater than 92% in a wavelength range from about400 nm to about 700 nm.
 15. The discharge vessel of claim 14 wherein thetotal transmittance is greater than 95%.
 16. The discharge vessel ofclaim 14 wherein the ceramic body contains about 200 ppm to about 800ppm MgO.
 17. The discharge vessel of claim 14 wherein the ceramic bodyhas a discharge cavity with a wall thickness of about 0.8 mm.
 18. Thedischarge vessel of claim 14 wherein submicron-grained alumina has anaverage grain size of about 0.5 microns.